1. Field of the Invention
The present invention relates to a voltage-controlled oscillator, and more particularly to a voltage-controlled oscillator having a control circuit for applying an input voltage to vary an output frequency from an oscillation circuit.
2. Description of the Related Art
One conventional voltage-controlled oscillator is illustrated in FIG. 1 of the accompanying drawings. As shown in FIG. 1, the conventional voltage-controlled oscillator, generally designated by the reference numeral 1, has a control circuit 2 and an oscillation circuit 3 which comprises a plurality of differential amplifiers successively connected in a ring pattern.
Specifically, the oscillation circuit 3 comprises three oscillation units 4, 5, 6 each comprising a differential amplifier. Each of the oscillation units 4, 5, 6 has five transistors 11, 12, 13, 14, 15.
The transistor 11, which is a first controlled load transistor, and the transistor 12, which is a second transistor, are connected in series with each other, making up a first series-connected circuit 16. The transistor 13, which is another first controlled load transistor, and the transistor 14, which is another second transistor, are connected in series with each other, making up a second series-connected circuit 17. The first series-connected circuit 16 and the second series-connected circuit 17 are connected parallel to each other.
The transistor 15, which is a current control transistor, is connected as a current source to one of the terminals of the series-connected circuits 16, 17 that are connected parallel to each other. A DC power supply 22, 23 is connected across the oscillation units 4, 5, 6.
Each of the first transistors 11, 13 comprises a p-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Each of the second transistors 12, 14 and the current control transistor 15 comprises an n-channel MOSFET.
The oscillation units 4, 5, 6 are successively connected in a ring pattern.
Specifically, a midpoint 18 between the transistors 11, 12 of the first series-connected circuit 16 of the first oscillation unit 4 is connected to the gate of the second transistor 12 of the first series-connected circuit 16 of the second oscillation unit 5.
A midpoint 19 between the transistors 13, 14 of the second series-connected circuit 17 of the first oscillation unit 4 is connected to the gate of the second transistor 14 of the second series-connected circuit 17 of the second oscillation unit 5.
The second oscillation unit 5 and the third oscillation unit 6 are similarly connected to each other, and the third oscillation unit 6 and the first oscillation unit 4 are similarly connected to each other.
In the connection between the third oscillation unit 6 and the first oscillation unit 4, however, the midpoint 18 of the first series-connected circuit 16 is connected to the second transistor 14 of the second series-connected circuit 17, and the midpoint 19 of the second series-connected circuit 17 is connected to the second transistor 12 of the first series-connected circuit 16.
Oscillated output terminals 20, 21 are connected to lines between the first and second oscillation units 4, 5. An output circuit (not shown) for generating a single rectangular pulse from two oscillated signals is connected to the oscillated output terminals 20, 21.
The control circuit 2 connected to the oscillation circuit 3 comprises a plurality of transistors 31-38 and has an input terminal 39 for a control voltage.
The control circuit 2 is connected to the gates of the first controlled load transistors 11, 13 and the current control transistors 15 of the series-connected circuits 16, 17 of the oscillation units 4, 5, 6.
The transistors 36, 38 of the control circuit 2 serve as drive transistors for outputting control signals to the oscillation circuit 3, and have gates and drains connected to each other.
Each of the transistors 32, 33, 35, 38 comprises a p-channel MOSFET, and each of the transistors 31, 33, 36, 37 comprises an n-channel MOSFET.
In the voltage-controlled oscillator 1 of the above structure, the oscillation units 4, 5, 6 each comprising a differential amplifier are successively connected in a ring pattern. The oscillation units 4, 5, 6 can output an oscillated signal from the oscillated output terminals 20, 21, and the frequency of the oscillated signal can be varied by a voltage applied to the control circuit 2.
Specifically, when the control voltage applied to the input terminal 39 of the control circuit 2 is varied, voltages applied to the gates of the transistors 11, 13, 15 of the oscillation units 4, 5, 6 are varied. Therefore, currents flowing through the transistors 11, 13, 15 are varied, varying the frequency of the oscillated signal outputted from the oscillated output terminals 20, 21.
By thus varying the voltages applied to the gates of the first controlled load transistors 11, 13 and the current control transistors 15 of the oscillation units 4, 5, 6, the currents flowing therethrough are varied to vary the frequency of the oscillated signal.
The voltage-controlled oscillator 1 can thus control the frequency of the oscillated signal with the inputted control voltage.
In the voltage-controlled oscillator 1, the transistors 11-15 of the oscillation circuit 3 operate in the saturated range. Therefore, if the transistors 11-15 of the oscillation circuit 3 and the drive transistor 38 in the output stage of the control circuit 2 have their transistor sizes and threshold voltages different from design values, then the oscillated signal may be stopped when the operating points of the oscillation units 4, 5, 6 change.
For example, the oscillating operation of the oscillation circuit 3 was simulated with different gate sizes of the drive transistor 38 of the control circuit 2 which is connected directly to the oscillation circuit 3. It was confirmed from the results of the simulation that, as shown in FIG. 2, when the gate size was increased 10%, the oscillated signal was reduced in level with time until the oscillating operation was stopped.
It is accordingly expected that the oscillating operation of the actual voltage-controlled oscillator may not be stabilized, but may be stopped, if gate sizes and threshold voltages of some transistors are changed due to manufacturing variations from design values.